This invention relates to high voltage solid state relays and more specifically relates to a novel relay circuit employing power semiconductor devices such as a triac or anti-parallel connected thyristors in which turn-on of the power devices is ensured under any inductive load and in which the load voltage is applied only to the power devices and a few associated components thereof, but is not applied to the control components of the relay.
Solid state relay circuits are well known and a typical solid state relay is shown in U.S. Pat. No. 3,723,769 entitled "Solid State Relay Circuit with Optical Isolation and Zero-Cross Firing" in the name of Collins, assigned to the assignee of the present invention. The solid state relay shown in the above patent employs anti-parallel connected thyristors or a triac in the main load circuit. Thyristor or triac firing is obtained in response to the turn-on of a pilot thyristor connected to the main power thyristor gate circuits. The power for operating the control circuit and the pilot thyristor is derived from the output circuit. Thus, the control components including the pilot thyristor must have voltage withstand capability high enough at least to withstand the voltage of the output circuit.
For very high voltage circuits, relatively expensive control components are needed. By an extremely high voltage a-c circuit is meant a circuit in which the RMS voltage is as high as about 480 volts at 50-60 Hz. Relay applications dealing with voltage of about 480 volts RMS are frequently encountered in both the United States and abroad and sometimes produce a peak voltage of, for example, 1200 volts which must be withstood by the individual components. Moreover, when circuits of this type are used to control very highly inductive circuits, the relay may not turn on as desired, upon the firing of the pilot thyristor, or the relay may "half-wave", in which only one of two anti-parallel connected thyristors is turned on.